Fertiliser is kind of a mystery to many beginner gardeners. What are those three numbers on the packet? Why are they important? In this post, I’ll be answering both of these questions.
What are those three numbers on the packet?
The three numbers featured on a packet of fertiliser (usually formatted something like 24-8-16) refer to an N-K-P ratio. N-K-P ratios outline what percentage of each of the three major plant macronutrients is present in the fertiliser, and more importantly, the ratio of the three.
The first number is nitrogen (N), the second is phosphorus (K) and the final one is potassium (P). An N-K-P ratio of 24-8-16 means that the fertiliser is 24% nitrogen, 8% phosphorus and 16% potassium. The remainder of the fertiliser is made up of trace elements, which are supplements that your plant doesn’t need in order to function, but provide a helpful boost, as well as other solids used to make the fertiliser but are not of importance to plants themselves.
What does each of the three do?
Nitrogen has the effect of encouraging vegetative growth on plants. Basically, it allows plants to create larger leaves more quickly, which in turn help it to photosynthesise and generate glucose, which it uses as energy. In turn, this furthers its ability to grow larger structures more quickly. This compounding growth effect is why larger plants have greater survivability and grow faster than smaller plants.
Phosphorus is important in the production of flowers. This is why fertilisers that focus on encouraging flowering in plants are higher in phosphorus in proportion to the other two elements. In fact, orchids require two different types of fertiliser: a plant food fertiliser which is higher in nitrogen, and a bloom booster, which is higher in phosphorus. In terms of orchid fertilisers, I always use the Manutec orchid fertilisers, and they consistently deliver results.
Potassium is responsible for a plant’s constitution for a number of reasons. It regulates the opening and closing of stomata, (think of them as plant pores), which in turn regulates their ability to cope with changes in temperature and humidity. The opening and closing of stomata is also implicated in photosynthesis. Potassium is also important to root health.
In general, houseplant fertilisers should have higher nitrogen, lower phosphorus and a potassium value somewhere in the middle. Brands like Osmocote and MiracleGro follow this rule in their houseplant fertilisers. However, not all effective fertilisers follow this formula. For example, EarthPod organic fertilisers have a 2-2-2 ratio and work as slow release fertilisers. Many have had great experiences with the fertiliser, despite its unorthodox N-K-P ratio. What fertiliser you use depends on the kind of growth you want to see on your plants. Those who grow for flowers may elect to start fertilising with a high phosphorus fertiliser when blooming season comes around.